Trylobyte
starting to spend too much on rocks
Member since October 2003
Posts: 101
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Post by Trylobyte on Jan 10, 2004 0:37:01 GMT -5
I've noticed on some of my tumbled rocks that I get a weird "canals of Mars" effect on the surface of the stone following polish. It seem that there is a tiny elevation difference between the canals (which appear more shiny and perhaps a few tens of thousanths of an inch wide) and the surrounding space (which is slightly less shiny and probably just a few 10,000ths deep). Generally the entire stone has this effect, and it's noticeably visible under strong light.
I've seen this on jasper, and some of the black or grey marbled beach rock that normally polishes nicely.
Is this "undercutting"? If so, what causes it, and how do I avoid it.
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Post by docone31 on Jan 10, 2004 20:32:23 GMT -5
The effect you are experiencing is the difference in mohs between different portions of the stone. Especially in jaspers. If you wish to avoid this in these stones, either grind below the different layers, or do not tumble. Cutting as a cabachon will reduce this tendancy. It will still be there, just better controlled. The softer will always abrade before the harder, even with diamond cutting. Faceting maintains a rigid plane of cut but even faceting, it still occurs. It can be seen under a loupe.
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